1. Field of the Invention
The invention relates to devices which utilize fiber optic cables. More specifically, the invention relates to methods and apparatuses which manage or organize the fiber optic cables in use in such devices.
2. Description of the Related Art
Modern computer and telecommunications networks are constantly growing more complex and have an ever-expanding need for bandwidth (the ability to accept, process, and/or transmit information). Many of the components used in such networks utilize optical transceivers and optical fibers as the means of communicating among and within the various components.
One of the ways that optical network components have become more complex over the years is through an increased density of optical transceivers and fibers. Typically, the optical fibers employed in a telecommunications device are mounted at one end on a circuit board or line module of some sort; the other end of the fiber exits the device to be connected to other devices or otherwise hook into a network. As fiber optic cables enter a circuit board through the front of the faceplate, another cable may attach to the rear of the faceplate at a bulkhead connector.
In many applications, a multi-fiber ribbon is employed instead of single incoming fibers. The incoming fiber ribbon can consist of up to 24 individual optical fibers. On the rear side of the faceplate of the line module, a Y-cable is typically used to connect the circuit board to the backplane of a telecommunications routing device. The Y-cable will branch off into two smaller ribbons; in the case of a 24-fiber ribbon, it will branch off into two 12-fiber branches, each branch terminating in a connector head.
If the end of one of the branches of the Y-cable is damaged, or even if only one of the 12 fibers in the connector head is damaged, the whole connector head must be severed, and that branch must be re-terminated. Typically, one must cut approximately xc2xe of an inch of cable off to effect a re-termination repair. It is not cost effective to throw out an entire Y-cable if only one branch sustains minimal, repairable damage. As a result, some cables are a little shorter than others, and in some Y-cables of this type, one branch may be shorter than the other branch.
Most modules or circuit boards have multiple inputs for fiber ribbons on the faceplate and thus must accommodate multiple fiber ribbons internally as well. For example, the SMX module sold by Ciena Corporation (the assignee of the instant invention) requires up to six 24-fiber ribbons. If Y-cables are employed internal to the faceplate, up to twelve smaller ribbons will be passing directly over the circuit board.
Having numerous cables passing over a circuit board can have deleterious effects on the performance of the system. First, the devices in which these circuit boards are employed are typically air cooled with the air flowing across the circuit board. If multiple fiber ribbons are haphazardly coiled on the circuit board, they will block or deflect the cooling air, and the components will not be cooled effectively. Also, if the fiber ribbons are not taut, the air flow may cause the slack fibers to vibrate. Vibration of the optical fibers against the circuit board can damage the circuit board, damage the fibers, and/or disrupt the flow of data through the fibers.
The conventional way to manage slack in multiple cable situations, such as in splice trays, patch panels, or wall mount enclosures, is by wrapping the cables around a spool or a ring. An example of this configuration is a product sold by AMP as part number 492440-1. In this product, 12 incoming fibers are wrapped around a central spool on the circuit board.
Such a configuration is not practical for fiber ribbon applications or many other applications. First, the fiber ribbon has a minimum bend radius which might be compromised were the ribbon to be wrapped around a spool, especially since each individual fiber would be bent at a slightly different radius, and even the same fiber would be bent differently as it wrapped around itself. There is also a great danger of twisting the ribbon as it is wound around the spool. Multi-fiber ribbon, for example, has a maximum twist specification of ten 180xc2x0 twists per yard or so of ribbon, or approximately 90xc2x0 in a minimum of about 2 inches of ribbon. Uncontrolled or excessive twisting of a ribbon could easily harm the optical fiber components therein, especially since they will also be bent. Further, it would be difficult to accommodate the possible variations in length of the branches of the fiber ribbons. Regardless of what type of fiber or fiber ribbon is employed, the spool would also interfere with the air cooling of the circuit board, board mounted components, and the overall device. Finally, it is much more difficult to service and/or replace a specific cable when several are wrapped around a single spool. In the case of the Ciena SMX module, 12 fiber ribbons would be wrapped around a spool, a configuration which would prove inconvenient to service.
The invention is a method and system of managing slack in fiber optic cables connected to a circuit board. The fiber optic cables are supported a vertical distance away from the circuit board each with at least one point of contact away from the circuit board. The point of support away from the circuit board is not colinear with the two points at which the cable is attached to the circuit board. Support is preferably provided in a plurality of points of support on a radius guide, preferably two radius guides. The radius guides each have a leading edge and a trailing edge for supporting the cable and a curved central portion therebetween around which the cable is bent. The cables are preferably tensioned between at least two of the points of support so that cables of differing lengths are all relatively taut over at least a portion of the lengths of the cables to prevent movement during shipping or to prevent vibration that would otherwise be caused by air flow over the circuit board.
Preferably, a tensioning assembly is provided contacting the cable at a point between the first and second radius guides providing tension to the cable. The tensioning assembly may include a leaf spring which is biasable against the cable and may be attachable to one of the radius guides. In the alternative, the tensioning assembly may be attachable to the circuit board above one of the radius guides. As another alternative, a single, larger radius guide could be employed, and the tension could be applied between the leading and trailing edges of that larger radius guide.
By moving the fiber optic ribbons (or cables) off of the circuit board of a module, air flow across the board remains unimpeded, and the components thereon can be cooled properly. Also, by keeping the ribbons in relatively straight lines instead of wrapping them around a common spool, the cables are more easily serviced. In addition, since the invention preferably keeps the ribbons taut, the ribbons will not vibrate or move around when air is blown over the circuit board or when the module is shipped, thereby helping to safeguard the integrity of the optical fibers in the ribbons.
In another aspect of the invention, the cable or ribbon is twisted only at a predetermined portion of the ribbon and only a predetermined angle, and the ribbon is prevented from being twisted anywhere else along its length. The inventive method and system prevent uncontrolled twisting of the fiber optic ribbons since they are preferably intentionally twisted only 90xc2x0, and that twist occurs wholly apart from where the ribbons are bent.